The present invention relates to surge or pulse suppression in fluid flow systems and, in particular, to gas analyzer systems, such as analyzers for automotive exhaust emissions.
In recent years government-mandated exhaust emissions standards for automotive internal combustion engines have resulted in various programs for testing or analyzing automotive exhaust emissions for compliance with the standards. These programs commonly involve the use of gas analyzer systems for analyzing the contents of exhaust emissions. Such gas analyzers commonly employ a pump for moving the stream of emissions through a conduit to an analysis apparatus, which commonly includes one or more sensors for detecting the presence of different constituents of the exhaust emissions. One such sensor is a nitric oxide (NO) sensor, which is in the nature of an electrochemical cell for measuring the quantity of nitric oxide in the exhaust emissions. Certain NO cells are very sensitive to pulsations in the gas stream incident on the sensor, and such pulsations can adversely affect the accuracy of the readings obtained by the sensor. Thus, pulsations, such as those introduced by the pump in the gas analyzer system, can have an adverse affect on the performance of the NO cell.
It is known to damp pulsations in fluid streams by effectively filtering the frequency of the pulsations. In a gas analyzer, the pulsations have a frequency which is related to the RPM of the pump. Such a filter may include a flow restricter, which is the acoustical equivalent of an electrical resistor, and an expansion chamber, which is the acoustic equivalent of an electrical capacitor. It has been found that it is possible to achieve effective damping of the pump pulsations by either (a) increasing the resistance by increasing the flow restriction, or (b) increasing the volume of the expansion chamber. The relatively large expansion chamber needed to effectively damp the pulsations takes a long time to fill, which leads to a substantial increase in the response time of the measuring system. Most regulations governing gas analyzer systems for automotive exhaust emissions specify maximum response times, and these mandated maximum response times may be exceeded with the large expansion chambers required to effectively damp the pulsations in the gas analyzer system.